Quote:
Originally Posted by brian nordstrom
When was the last time you viewed Sirius at 400x with a 102mm f10 achro
|
The first time was 45 years ago, through an 1880 Thomas Cooke 4.25" refractor, f/16 achro, with a truly exquisite objective. The views of Jupiter and Saturn through that scope are unforgettable. And it split Sirius, easily.
Ever since then I have been a high-magnification junkie mainly interested in lunar & planetary and to some extent double stars, as arguably the most severe optical test.
While I had access to the 9" Oddie refractor at Mt Stromlo - and pushed it to 900X while trying a few of the double stars in various catalogs and rapidly realised that in the 18th and 19th centuries, even though those old antique refractors were only 4" - 6" aperture:
a) didn't have modern glasses,
b) didn't have AR coatings,
c) had very limited eyepieces (try a Ramsden or Huygens and see how you feel about that)
... the records of the stars they were able to split indicate some of the scopes had truly exquisite objectives, even by modern standards.
Another clue lies in the notes from Dawes (the 'Dawes limit"), who concluded it is necessary to use magnification around 80X per inch to definitively split doubles at the limit of what the telescope can resolve. But for many modern refractors the useful limit is barely half that because the optics are frankly only "average", being designed to satisfy the ¼ wave criterion.
There really is a reason why premium old-school achro refractors were f/15-f/20, despite the massive equatorial mounting needed.
There really is a reason why people fork out $10k for an AP 130 GT. But aperture does rule and there are rare mythical beasts such as refractors with LZOS optics, or the premium big maks such as my MK91, or the TEC and AP 10" maks - if you can wait 20 years for one. The dinky little Takahashi's are toys in comparison.